Comparator unit for a flash analog-to-digital converter

ABSTRACT

A high speed comparator unit for a flash A/D converter in which a bank of comparator units compare simultaneously an analog input voltage with equally spaced reference voltages, and an encoder ROM produces digital signals based on the comparator unit&#39;s outputs. The comparator unit includes a two-stage cascode configuration and a level shifter configuration which effectively reduces the miller-effect of the comparator unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a comparator unit for a flash A/D converter in which a bank of comparator units compare simultaneously an analog input voltage with equally spaced reference voltages, and an encoder ROM produces digital signals based on the comparator unit's outputs. More particularly, the present invention relates to a high speed comparator unit including a miller-effect reducer which reduces the miller-effect of latch transistors to increase comparison speed.

2. Description of the Prior Art

The flash A/D converter is the fastest of all converter systems. Conversion speed is limited by the speed of a comparator unit.

As shown in FIG. 1 (PRIOR ART), a conventional flash A/D converter comprises a plurality of comparator units 5 for simultaneously comparing an analog input voltage V_(in) with equally spaced referenced voltages V_(ref). A voltage divider of resistors R_(n1), R_(n2), . . . R_(n2) m disposed between supply voltage V_(TOP) and V_(BTM) are connected to an inverting terminal (-) of each comparator CP₁, CP₂, . . . CP₂ m, and an analog input voltage V_(in) is connected to a noninverting terminal (+) of each comparator CP₁, CP₂, . . . CP₂ m. Output terminals of the comparators CP₁, CP₂, . . . CP₂ m are connected to encoder ROM 10 which converts the analog input voltage V_(in) to a digital signal D₁ . . . D_(m). Therefore, the comparators CP_(1-CP) ₂ m compare the analog input voltage V_(in) with the reference voltages V_(ref) generated by the voltage divider and the encoder ROM 10 produces corresponding digital signals D₁ . . . D_(m) based upon these comparisons. As shown in FIG. 1, generally there are numerous comparators CP₁, CP₂, . . . CP₂ m, and thus, numerous comparator units 5 are needed for a conventional flash A/D converter.

FIG. 2 (PRIOR ART) shows a circuit diagram of a conventional comparator unit 5 comprising clock driven transistor Q₁₂ and Q₁₃, transistors Q_(1-Q) ₃, Q₁₀ and Q₁₁, latch transistor Q₈ and Q₉, and resistors R₁ -R₃ and R₅.

When clock CK goes high, transistor Q₁₃ turns on. If analog input voltage V_(in) is higher than reference voltage V_(ref), a larger portion of current I₂ flows through transistor Q₁₀ than transistor Q₁₁, thereby producing a larger voltage drop across resistor R₂ than resistor R₃. Accordingly, the voltage at the collector of transistor Q₁₀ is lower than the voltage at the collector of transistor Q₁₁. Since the voltage drop across resistor R₃ is small (i.e., current I₂ is smaller along the transistor Q₁₁ side of the comparator), likewise the voltage drop across R₁ is small because current I₂ is small (VB1 is high, thus transistor Q₂ is on). Thus, with a small current I₂ at the collector of transistor Q₂ (or enable terminal E_(n+1), which does not have current flowing from adjacent comparator CP_(i+1) along its corresponding transistor Q₁ and enable terminal E_(n-1)), output voltage V_(out) at the emitter of transistor Q₃ is a high level of V_(CC-V) _(BE) (Q₃).

On the other hand, if analog input voltage V_(in) is lower than reference voltage V_(ref), a larger portion of current I₂ flows through transistor Q₁₁ than transistor Q₁₀, thereby producing a larger voltage drop across resistor R₃ than resistor R₂. Accordingly, the voltage at the collector of transistor Q₁₀ is higher than the voltage at the collector of transistor Q₁₁. Since the voltage drop across resistor R₃ is large (i.e., current I₂ is larger along the transistor Q₁₁ side of the comparator), likewise the voltage drop across R₁ is larger because current I₂ is larger (VB1 is high, thus transistor Q₂ is on). Thus, with a larger current I₂ at the collector of transistor Q₂ (or enable terminal E_(n+1)), output voltage V_(out) at the emitter of transistor Q₃ is a low level of V_(CC) -I₂ ×R₁ -V_(BE) (Q₃).

When CK goes high, transistor Q₁₂ turns on thereby allowing current I₂ to flow through latch transistors Q₈ and Q₉. Since the collector voltages of transistors Q₁₀ and Q₁₁ are connected to the bases of latch transistors Q₈ and Q₉, during latching the collector voltages of transistors Q₈ -Q₁₁ maintain their pre-state voltages, thus maintaining the voltage difference between the collectors of latch transistors Q ₈ and Q₉. If analog input voltage V_(in) is larger than reference voltage V_(ref), current I₂ flows through transistor Q₁₂, latch transistor Q₉, resistor R₂, and transistor Q₁ to enable terminal E_(n-1) and decreases the collector voltage of latch transistor Q₉ by I₂ ×R₂ lower than latch transistor Q₈. On the other hand, if analog input voltage V_(in) is smaller than reference voltage V_(ref), current I₂ flows through transistor Q₁₂, latch transistor Q₈, resistor R₃, and transistor Q₂ and decreases the collector voltage of latch transistor Q₈ by I₂ ×R₃ lower than latch transistor Q₈.

By this latching operation, when analog input voltage V_(in) is larger than reference voltage V_(ref) of comparator CP_(i) and smaller than reference voltage V_(ref) of comparator CP_(i+1), only comparator CP_(i) provides a high level output voltage V_(out) while the other comparators produce low level output voltages.

However, in a conventional comparator unit, resistors R₂ and R₃ are connected between the emitters of transistors Q₁ and Q₂ and the collectors of latch transistors Q₈ and Q₉ causing a loop (for examples, the loop for latch transistor Q₉ comprises the collector of latch transistor Q₉, resistor R₂, the emitter of transistor Q₁, the bases of transistors Q₁ and Q₂, the emitter of transistor Q₂, resistor R₃, the base of latch transistor Q₉) to have a gain which increases the miller-capacitance between the collectors and bases of latch transistor Q₈ and Q₉, thus lowering the speed of the comparator unit 5.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a comparator unit for a flash A/D converter comprising a miller-effect reducer which can reduce the miller-effect of a latch transistor to increase the comparison speed.

Accordingly, a comparator unit of the present invention comprises clock driven transistors, transistors for comparing analog input voltages with references voltages, latch transistor configuration, and a cascode transistor configuration. The cascode transistor configuration is connected to the latch transistor configuration and operates to reduce a miller-capacitance effect to increase the comparison speed.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the appended claims. The present invention itself, however, as well as other features and advantages thereof, will best be understood by reference to the following detailed description of the preferred embodiment, read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of a conventional flash A/D converter;

FIG. 2 is a circuit diagram of a conventional comparator unit of a flash A/D converter; and

FIG. 3 is a circuit diagram of a comparator unit for a flash A/D converter according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Like elements appearing in FIGS. 2 and 3 are designated with like reference numerals.

As shown in FIG. 3, a comparator unit 5 of the present invention comprises clock driven transistors Q₁₂ and Q₁₃, transistors Q₁ -₃, Q₁₀ and Q₁₁, latch transistors Q₈ and Q₉, and resistors R₁ -R₃ and R₅ according to a conventional comparator unit and further comprises miller-effect reducer 15 coupled between latch transistors Q₈ and Q₉ and resistors R₂ and R₃, and resistor R₄.

Two cascode transistors Q₁₄ and Q₁₅ are connected between resistor R₂ and the collector of latch transistor Q₉ and between resistor R₃ and the collector of latch transistor Q₈, respectively. Positive feed back signals from the collectors of transistors Q₁₄ and Q₁₅ are supplied to the bases of latch transistors Q₈ and Q₉ through a level shifter transistor configuration of transistors Q₄, Q₆ and Q₅, Q₇, respectively. The remaining structure and elements correspond to those previously described in FIGS. 1 and 2.

When clock CK goes high, the comparator compares analog input voltage V_(in) with the reference voltage V_(ref). When analog input voltage V_(in) is larger than reference voltage V_(ref) a larger portion of current I₂ flows through transistor Q₁₀ than transistor Q₁₁. The current flowing through transistor Q₁₀ flows through transistor Q₁₄, resistor R₂, and transistor Q₁ from E_(n-1), while the current flowing through transistor Q₁₁ flows through transistor Q₁₅, resistor R₃, transistor Q₂, and resistor R₁ from V_(CC). Since current I₂ is larger in transistors Q₁₀, Q₁₄ than transistors Q₁₁, Q₁₅, the collector voltage of transistor Q₁₄ is lower than the collector voltage of transistor Q₁₅. These collector voltages propagate to the emitter voltages of transistor Q₆ (base voltage of transistor Q₈) and transistor Q₇ (base voltage of transistor Q₉) via level shifter transistors Q₄, Q₆ and Q₅, Q₇, respectively. Accordingly, the emitter voltage of transistor Q₆ is lower than transistor Q₇. The remaining operation of transistor Q₁ -Q₃ and derivation of voltage V_(out) is the same as described in FIG. 2.

Similarly, when the analog input voltage V_(in) is smaller than reference voltage V_(ref), a larger portion of current I₂ flows through transistor Q₁₁ than transistor Q₁₀. Therefore, the collector voltage of transistor Q₁₄ is higher than the collector voltage of transistor Q₁₅ and the emitter voltage of transistor Q₆ (base voltage of transistor Q₈) is higher than that of transistor Q₇ (base voltage of transistor Q₉).

When clock CK goes high (i.e. latching begins), current I₂ flows through transistor Q₁₂ thereby turning on transistors Q₈ and Q₉. If in the prelatch state condition analog input voltage V_(in) is larger than reference voltage V_(ref) (base voltage of transistor Q₈ is lower than that of transistor Q₉), the voltage difference between these voltages is amplified along positive feed back loop Q₈ - Q₁₅ - R₃ - Q₅ - Q₇ - Q₉ - Q₁₄ - R₂ - Q₄ - Q₆. Current I₂ flows through transistor Q₁₂, transistor Q₉, transistor Q₁₄, resistor R₂, and transistor Q₁ so that the base voltage of transistor Q₈ is lower than that of transistor Q₉ by I₂ x R₂.

On the other hand, if the pre-state condition is such that analog input voltage V_(in) is smaller than reference voltage V_(ref) (base voltage of transistor Q₈ is higher than that of transistor Q₉), the voltage difference between these voltages is amplified along positive feed back loop Q₈ - Q₁₅ - R₃ - Q₅ - Q₇ - Q₉ - Q₁₄ - R₂ - Q₄ - Q₆. Current I₂ flows through transistor Q₁₂, transistor Q₈, transistor Q₁₅, resistor R₃, transistor Q₂, and resistor R₁ from V_(CC) so that the base voltage of transistor Q₈ is higher than that of transistor Q₉ by I₂ ×R₃.

According to the present invention, a miller-effect reducer 15 has a level shifter transistor configuration and a two-stage cascode transistor configuration. Transistors Q₁, Q₂ and Q₁₄, Q₁₅ are connected in a two-stage cascode arrangement and constant voltage VB2 is applied to the bases of transistors Q₁₄ and Q₁₅. Further, the base of transistors Q₈ and Q₉ are connected to emitters Q₆ and Q₇, respectively (rather than the base of transistor Q₈ being connected to the collector of transistor Q₉ and vice versa).

The level shifter transistor configuration Q₄ -Q₇ lowers the base voltages of transistors Q₈ and Q₉. As a result, transistors Q₈ and Q₉ are prevented from saturation because the collector voltages of transistors Q₁₄ and Q₁₅ are higher than those of transistors Q₈ and Q₉. Thus, amplified signals along resistors R₂ or R₃ propagate through level shifter transistors Q₄, Q₆ or Q₅, Q₇, respectively, thereby reducing the miller-capacitance effect and enhancing the speed of the comparator unit 5.

The present invention is advantageous over conventional comparator units in that a one-stage cascode transistor configuration Q₁ and Q₂ connected to latch transistors Q₈ and Q₉ through resistors R₂ and R₃ (FIG. 2) cannot reduce the miller-capacitance of latch transistors Q₈ and Q₉. However, in the present invention, a second cascode stage transistors Q₁₄ and Q₁₅ is placed between resistors R₂ and R₃ and latch transistors Q₈ and Q₉ wherein a positive feed back loop from the collectors of transistors Q₁₄ and Q₁₅ contains the level shifter transistor configuration. This second cascode stage and the positive feed back loop effectively reduce the miller-capacitance effect and the conversion speed of the flash A/D converter can be enhanced through the enhancement of the speed of the comparator unit 5. 

What is clained is:
 1. A comparator unit for a flash A/D converter having a plurality of comparator units and an encoder ROM, each said comparator unit comprising:switching transistors which switch conductive states based upon a clock input signal; comparing transistors having a single output for comparing an input voltage to a reference voltage, said comparing transistors having their output connected to one of said switching transistors; latching transistors for latching the output voltage of the comparing transistors, said latching transistors being connected to said comparing transistors and said switching transistors and latch based upon the input clock signal; miller-capacitance reducing means for reducing a miller capacitance effect of said latching transistors, said miller effect reducing means comprising a pair of transistors having a constant d.c. power input and whose output is connected to said latching transistors and said comparing transistors; first and second signal transistors being connected to said miller-capacitance reducing means and said latching transistors for generating a signal to an output means; and said output means outputting said compared output voltage to an encoder ROM based upon the comparison of the input voltage and the reference voltage.
 2. The comparator unit according to claim 1 wherein said miller-capacitance reducing means comprises level shifting means coupled between said latching transstors and said first and second signal transistors.
 3. The comparator unit of claim 2 wherein said level shifting means comprises a pair of transistors in cascade arrangement connected between the first signal transistor and a latch transistor and a pair of transistors in cascade arrangement connected between the second signal transistor and a latch transistor.
 4. A comparator unit for a flash A/D converter comprising:a first switch transistor having a base connected to a clock signal, a collector, and an emitter; a second switch transistor having a base connected to an inverted clock signal, a collector, and an emitter; a pair of comparing transistors having their bases connected to voltage signals to be compared, collectors, and emitters connected to the collector of said first switch transistor; a first latch transistor having a base, a collector connected to the collector of one of said comparing transistors, and an emitter connected to the collector of said second switch transistor; a second latch transistor having a base, a collector connected to the collector of another of said comparing transistors, and an emitter connected to the collector of said second switch transistor; a first resistor and a second resistor each having a first terminal and a second terminal; a first signal transistor having a base connected to a first voltage source, a collector connected to an enable line of a first adjacent comparator unit, and an emitter connected to a second terminal of said first resistor; a second signal transistor having a base connected to the first voltage source, a collector connected to an enable line of a second adjacent comparator unit, and an emitter connected to a second terminal of said second resistor; a third resistor having a first terminal connected to the collector of said second signal transistor and a second termial connected to a voltage supply; a third signal transistor having a base connected to the collector of said second signal transistor, a collector connected to the voltage supply, and an emitter connected to an output; and a miller-capacitance reducer, having a plurality of transistors, each of the transistors being connected to at least one of the collectors of said comparing transistors, the base of said first and second latch transistors, and the first terminal of said first and second resistors.
 5. A comparator unit according to claim 4 wherein said miller-capacitance reducer comprises:a pair of cascode transistors having their bases connected to a second power source, their collectors connected to the second terminals of said first and second resistors, and their emitters connected to the collectors of one of said pair of comparing transistors and to one of said first and second latch transistors; and level shifting transistors coupled between the collector of one of said pair of cascode transistors and the bases of said first latch transistor and said second latch transistor.
 6. A comparator unit according to claim 5 wherein said second level transistors function as one of a resistor and a diode.
 7. A comparator unit according to claim 4 wherein said level shifting transistors comprise two pairs of transistors, a first pair coupled between one of said cascode transistors and the base of said first latch transistor, and a second pair coupled between another of said cascode transistors and the base of said second latch transistor, each pair comprising:a first level transistor connected to the respective collector of said cascode transistors; and a second level transistor connected to said first level transistor and the respective bases of said first latch transistor and said second latch transistor. 